But while officials and media originally speculated that the killings may have been gang-land related, by Thursday evening it was becoming clear that police and security services were treating them as terror related.

On Thursday evening counter terrorist operations were ongoing in two districts of the Stavropol region, both of which border Kabardino Balkaria, a republic in the Russian North Caucasus where a separatist Islamist insurgency continues to battle Russian forces.

The Federal Security Service, Russia’s main internal security agency, asked locals to remain indoors and if they were on the streets to exercise vigilance, looking out for abandoned vehicles in particular.

Earlier in the day local authorities called out the army and even local Cossacks to back up a high-visibility security presence across the region.

“In addition to law enforcement agencies, the Stavropol region is also now being patrolled by Cossack brigades,” regional governor Vladimir Vladimirov said in a statement on Thursday afternoon. “I have appealed to Defense Minister Sergei Shoigu for help in strengthening security in the region with the 49th Army.”

Reports that three insurgent fighters from Kabardino Balkaria had been named as likely suspects were unconfirmed on Thursday evening, as were earlier reports that that FSB officers had detained a woman wearing a suicide vest.

The attacks follow successive suicide bombings in Volgograd, about 400 miles from Sochi and major transport hub that acts as a gateway to much of the Caucasus.

Russia’s security forces are on a state of high alert ahead of the opening of the Sochi Winter Olympics on February 7, which insurgent leaders have vowed to disrupt.

Some 40,000 police and troops backed by drones, surface to air missiles, and Cossacks have been deployed to form a so-called “ring of steel” around the host city, which sits at the Western end of the mountain chain at the centre of the insurgency.